<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    *{
        margin: 0;
        padding: 0;
    }
    .bigbox{
        width: 500px;
        height: 500px;
        border: 10px solid gray;
        margin: 100px auto;
        display: flex;
        justify-content: center;
        align-items: center;
    }
    .bigbox div{
        width: 50px;
        height: 50px;
        background-color: red;
        /* margin: auto; */
    }
</style>
<body>
    <div class="bigbox">
        <div></div>
    </div>
</body>
</html>